Ceramic articles for various uses have been fabricated by many techniques. In the manufacture of many ceramic articles, a "green" body is formed of ceramic powder, binder, and possibly other ingredients. The green body is then typically fired to remove the binder and sinter the ceramic powder to form the ceramic article. The shape of the sintered article generally corresponds to the shape of the green body. Various methods have been used to make green bodies of a desired shape. One of those methods is injection molding.
In injection molding, a mixture containing ceramic powder, a binder and possibly other ingredients is injected into a mold corresponding to the desired green body shape. Typically, the mixture is heated to lower its viscosity before molding. The mixture is allowed to harden in the mold, and the mold is then removed to recover the green body. The green body is then fired to remove the binder and sinter the ceramic powder.
Several methods have been used to form the mixture to be injected. In some processes, the ceramic powder and the binder are directly mixed together by kneading. Kneading methods generally suffer from poor dispersion of the binder and ceramic resulting in defects in the final structure. The binder systems used in the kneading technique generally require long binder burnout times. Most systems using kneading also have problems with defects such as knit lines due to relatively high molding pressures. While some attempts have been made to address the binder burnout issue by using exotic additives (e.g. U.S. Pat. No. 4,784,812), these solutions add extra cost and do not address other problems associated with methods using kneading.
The other principle approach to forming the mixture for injection involves the formation of a dispersion of ceramic powder in a dispersion medium which also contains the binder. In most dispersions, the dispersion medium is a solvent for the binder whereby the binder is dissolved and distributed among the ceramic particles. Once the dispersion is formed, some or all of the dispersion medium is removed by drying. In some cases, the dispersion is dried to form a free flowing powder. The dispersion method has worked best with systems using organic solvents. Unfortunately, organic solvents present environmental problems in the drying stage. Water soluble binders systems used previously have not exhibited good flow characteristics on injection. Thus, the problems of defects, knit lines, etc. still exist for these systems.
Thus, there remains a need for an effective injection molding system for making ceramic bodies.